Experimental support for physisorbed positronium at the surface of quartz

Abstract

We report temperature-dependent positronium (Ps) emission from a single crystal of ${\mathrm{SiO}}_{2}$ using a monoenergetic positron beam. Slow positrons (${e}^{+}$) from an electrostatic beam system were injected with variable energy (0--1600 eV) into the ${\mathrm{SiO}}_{2}$ target and Ps emission from the target surface was studied as a function of incident ${e}^{+}$ energy (E) as well as target temperature (T). Our data suggest a physisorbed Ps surface state which is temperature activated into a ``slow'' Ps emission with an activation energy of \ensuremath{\sim}0.15 eV. In addition, a large Ps yield (40% of the incident positrons are emitted as Ps at 400 eV) is observed even for T\ensuremath{\rightarrow}0, attributed to ``fast'' Ps produced by the bulk Ps formed within the ${\mathrm{SiO}}_{2}$ target and diffusing to the surface. From the Ps yield vs E we find a bulk Ps diffusion constant of 0.047\ifmmode\pm\else\textpm\fi{}0.013 ${\mathrm{cm}}^{2}$/sec. We also observe a slow ${e}^{+}$ reemission yield of (15\ifmmode\pm\else\textpm\fi{}2)% at 400-eV incident ${e}^{+}$ energy.